The present invention relates to skywave communication systems. By skywave, we mean that communication which is supported by ionized regions above the earth. Such communication is characteristic of high frequency (HF) but it also occurs at even higher frequencies. One such higher band is typically designated as very high frequency (VHF).
HF is a traditional approach for economical, non-line of sight communications. However, existing HF communication systems exhibit problems for reliable and economical use.
One problem concerns the behavior of the HF medium. The frequencies supported by the medium, the signal attenuation, and the noise level are all variable with time. General tendencies over long periods (daily, yearly and solar cycle periods) are known, but short term variations, say over a few minutes or few hours, occur in an apparently random fashion. In the past, an efficient use of the medium required operators with considerable skill and experience to recognize channel changes and to estimate what an appropriate remedial response might be. Skilled operators are a limited resource and there is a need for systems that can be effectively employed by any user. Also, as sophistication and transmission techniques are applied (such as frequency hopping for security purposes) limitations occur for even the best operators to manually operate and optimize the system. A second problem in the prior art concerns the need for interconnecting a number of HF nodes. If a given node is to be connected to "N" neighbors, then the node must have N radio terminals or it must share one or several radio terminals among the N neighbors. Each of these N radio links will individually exhibit the medium variations described above. Systems exist to accomplish sharing via scanning techniques that lock onto a calling node but it is desirable to develop a means for sharing that prevents blocking of signal transmissions, or in other words, is non-blocking.
The present invention attempts to overcome the limitations of the prior art by using time division multiple access (TDMA) to share a given radio at a given node among the links connecting to its neighbors. In the embodiment illustrated herein, a single radio located at each node is directly connected to four neighbors. However, the overall design structure accommodates other numbers of neighbors and permits the use of one or more radios at any of the nodes. The primary function of the system is to automatically establish and maintain connectivity between nodes of the system. When properly connected, the basic modem/radio terminal can accept and deliver messages in accordance with automated link management algorithms. Connectivity is established and maintained through a series of protocols that search a frequency spectrum, measure the channel parameters and adaptively match transmission parameters such as data rate to the channel. A channel maintenance protocol is activated on a routine basis or whenever significant changes are sensed in the measured channel characteristics. Connectivity is initiated with a synchronized link-up sequence in which transmission occurs on a predefined library of frequencies in the time slots associated with those neighbors for which connectivity is being sought. The receiving node reports on the link quality of the probes in the link-up sequence as received by that node using a synchronized response on its library of frequencies. As a result of this exchange, each node knows the frequency on which reception of its transmissions was optimum as well as the relative quality of alternate frequencies. At this point, an automated handshake procedure confirms the selection of the frequencies to be used for subsequent transactions. These are designated as "orderwire" frequencies. At the conclusion of this process, the link is declared connected and ready for traffic.
In this TDMA system, the activities on any given link, which is defined by a frequency/time slot, are independent from those occurring in other time slots. Thus, a node may be in some stage of the connect procedure with some of its neighbors, in traffic ready status with other neighbors and in maintenance protocols with still other neighbors.
The normal operation of the system presented herein employs synchronization by the participating nodes. A stable time reference is incorporated in each node with means to set this standard to a master reference such as radio station WWV of the National Bureau of Standards or a global positioning system (GPS) satellite or it may be provided via a downline technique from a master time station within the total system.
It is therefore an object of the present invention to provide an improved communication system using a multiplicity of nodes operating on a defined neighbor basis for transmitting information packets between neighbors to the eventual destination using adaptive frequency selection between nodes where all of the frequencies being used may potentially be different, and in accordance with conditions at that moment in time for optimum reception .